Porcine circovirus type 2 (PCV2), a member of Circoviridae family, genus Circovirus, is a small nonenveloped circular virus which was initially discovered in 1998. PCV2 is one of the two most prevalent pathogens encountered in the pig industry, the other being Mycoplasma hyopneumoniae (M. hyo). Swine infected with PCV2 exhibit a syndrome commonly referred to as Post-weaning Multisystemic Wasting Syndrome (PMWS). PMWS is clinically characterized by wasting, paleness of the skin, unthriftiness, respiratory distress, diarrhea, icterus, and jaundice. In addition to PMWS, PCV2 has been associated with several other diseases, including pseudorabies, porcine reproductive and respiratory syndrome (PRRS), enzootic pneumonia, Glasser's disease, streptococcal meningitis, salmonellosis, postweaning colibacillosis, dietetic hepatosis, and suppurative bronchopneumonia. The various clinical manifestations of PCV2 infection in pigs across the age groups has become known as porcine circovirus-associated disease (PCVAD), and are characterized by wasting and growth retardation. PRRS virus, Swine Influenza Virus (SIV), M. hyo, and other bacteria have been implicated as major co-factors in the development of PCVAD. PCVAD has continuously been a threat to the global swine industry, causing high economic losses.
PCV2 isolates are currently further subdivided into three genotypes: PCV2a, PCV2b, and PCV2c. PCV2 contains two major open reading frames (ORFs), which encode a protein associated with replication (ORF1, 945 nt), and the virus capsid (ORF2, 702 nt). PCV2 has undergone significant genetic variation in recent years. A newly emergent PCV2 mutant with an additional lysine (K) at the C-terminus of the ORF2-encoded capsid protein compared with classical PCV2a and PCV2b genotypes was isolated in 2008 from a serum sample from an aborted pig (Guo et al., 2010, Virology Journal 7: 273). In this newly emerging PCV2 mutant, a one-base deletion at position 1039 in the genomic sequence resulted in a mutation of the stop codon (from UAA to AAG) in ORF2, to give an ORF2 gene of 705 nt and a new stop codon (Guo et al., 2011, Virology Journal 8: 291). In addition, Knell et al. have reported previously that mutations could occur in the ORF2 gene, because a deletion (T) was found at position 1042 in the 1767 nt genome of one strain (GenBank no. AY713470), which led to elongation by one amino acid (lysine) in the C terminus of the ORF2-encoded capsid protein (Knell et al., 2005, Veterinary Microbiology 109: 169-177). Olvera et al. have also reported elongation by one lysine residue of the C terminus of the capsid protein due to a mutation in the stop codon of ORF2 (Olvera et al., 2007, Virology 357: 175-185). Additionally, a PCV2 strain termed “JSTZ”, with GenBank accession No. JQ413808, was detected and identified in stool samples of a piglet with severe diarrhea in China, and its complete 1767 nt genome was sequenced (Li et al., 2012, Journal of Virology (jvi.asm.org), p. 4716). Phylogenetic analyses based on the genome of PCV2 strain JSTZ and the ORFs of other Chinese PCV2 strains indicated that PCV2 strain JSTZ belonged to a novel genotype in China (Li et al., 2012, supra).
Guo et al. assessed the relative virulence of a PCV2 mutant strain termed PCV2b/rBDH or BDH (Gen Bank accession No. HM038017), which had been recovered in 2008 from a sample from an aborted pig with PMWS, and confirmed the greater virulence of the PCV2 mutant strain in piglets than that associated with the classical PCV2a and PCV2b genotypes (Guo et al., 2012, PLoS ONE (plosone.org), Vol. 7, Issue 7, e41463, 1-10). This PCV2 mutant strain demonstrated more severe signs compatible with PMWS, characterized by wasting, coughing, dyspnea, diarrhea, rough hair-coat and depression. Moreover, the pathological lesions and viremia, as well as the viral loads in lymph nodes, tonsils, and spleen, were significantly more severe for piglets challenged with the PCV2 mutant strain compared with those in the groups challenged with classical PCV2a and PCV2b. In addition, a significantly lower average daily weight gain was recorded in the group challenged with the PCV2 mutant strain than in the groups challenged with the prevailing PCV2a and PCV2b genotypes (Guo et al., 2012, supra).
Two PCV2 strains, US22625-33 and US22664-35, were recently identified in cases of suspected vaccine failure in PMWS-affected pigs in a production system located in the United States (Xiao et al., 2012, Journal of Virology (jvi.asm.org), Vol. 86, No. 22, p. 12469). The full genome of these two US strains was found to be comprised of 1767 nt, and the size of its ORF2 gene was 705 nt, encoding an ORF2 protein of 234 aa, which was one amino acid longer than that of common PCV2. Phylogenetic analysis with the nucleotide sequences of ORF2 of classical PCV2a and PCV2b strains suggested that both U.S. PCV2 strains US22625-33 and US22664-35 are closely related to PCV2b. Compared with classic PCV2b, a single base deletion within the ORF2 gene resulted in the addition of a single amino acid (lysine) to the C-terminus of the ORF2 protein. Further sequence BLAST and comparison showed that both U.S. PCV2 strains had a high level of identity (99.9%) with the PCV2 strain, BDH, found in China, and reported to be of increased virulence. One silent mutation (1677A→1677T) in ORF1 was found between BDH and the two U.S. mutant PCV2s. According to the new PCV2 genotype definition and nomenclature criteria (Cortey, et al., 2011, Vet. Microbiol. 149:522-523; Segales, et al., 2008, Vet. Rec. 162:867-868), all of these novel mutant PCV2 strains could be classified into genotype PCV2b, based on the phylogenetic analysis of the nucleotide sequence of the ORF2 gene (Xiao et al., 2012, supra).
In view of the reported increased virulence of the new PCV2b divergent, as well its presence in cases of suspected vaccine failures in the United States, what is needed is an efficacious vaccine against this new PCV2b divergent. Preferably, this vaccine will be compatible with other porcine antigens, such as M. hyo and PRRS virus.